Since the late 1950s and early 1960s, molecular biologists have learned to characterize, isolate, and manipulate the molecular components of cells and organisms: DNA, RNA, and proteins. The discovery of polymerase chain reaction in the 1970s enabled biologists to amplify DNA to quantities that could be manipulated. The isolation of restriction endonucleases in the 1970s significantly advanced molecular biology, allowing for the production of recombinant nucleic acids and proteins.
One of the most basic techniques of molecular biology used to study protein function is expression cloning, which was developed in large part using E. coli. In this technique, DNA coding for a protein of interest is cloned (using PCR and/or restriction enzymes) into a plasmid or other expression vector. This plasmid may have replication elements so that it is maintained in a host cell (e.g., an E. coli cell), special promoter elements to drive production of the protein of interest, and may also have antibiotic resistance markers to help follow the plasmid.
Expression cloning has allowed for the recombinant production of proteins, which avoids many of the difficulties and hazards of protein purification from natural sources. For example, the recombinant production of human proteins eliminates workers' exposure to human fluid and tissues, avoiding potential exposure to infectious agents such as viruses. In the case of many environmentally, industrially, and therapeutically useful proteins, recombinant manufacturing is frequently the only practical method for producing the amounts of protein required for use or sale. However, the requirements and needs for large scale recombinant production of proteins are distinct from those of small scale production as would occur, for example, in a research laboratory.